Endothelial cells are involved in the regulation of several functions in the cardiovascular system such as vascular tonicity, coagulation, angiogenesis, etc. Various agonists (bradykinin, ATP, etc) are capable of activating the endothelial cells. Little is known about how the electrical properties of endothelial cells are affected IN VIVO after agonist stimulation. The long term goal of the present study is to determine the effect of agonist stimulation on intracellular calcium homeostasis and in the changes on cell membrane potential and cell currents. Preliminary data indicates that different agonists induce diverse changes in the cell membrane potential. The effect of these agonists on the activation of a calcium-activate potassium channel indicate that this channel is modulated by two different G proteins, a PTX-insensitive G protein coupled to the bradykinin receptor increases channel activity at a constant intracellular calcium. On the other hand, a PTX-insensitive G protein coupled to the purinergic receptor decreases channel activity at a constant calcium. This dual regulation may be responsible for the oscillation in cell membrane potential observed with one of the agonists. All the proposed studies will be performed on freshly isolated vena cava isolated endothelial cells. Some of the studies will be performed IN SITU (without disrupting the endothelial monolayer). The simultaneous measurements of intracellular calcium, cell membrane potential and whole cell currents before and after agonist stimulation (using various agonists) will provide important information about cell signaling IN VIVO.